The present invention relates to an optical disk apparatus for reading out information recorded in an optical disk, in the form of a pit stream. More particularly, the invention relates to an optical disk apparatus which can read out information from both a high-density optical disk and a low-density optical disk, without necessity of using special optical elements.
Read-only optical disks for use as storage devices in computers or as media for recording audio and video signals have advanced in terms of their storage densities, from CD-ROMs to DVD-ROMs. A DVD-ROM is an optical disk based on the DVD-ROM standards. Various DVD-ROM drives have been developed to read out information from DVD-ROMs. The DVD-ROM drives have backward compatibility. In other words, each DVD drive can read out information not only from DVD-ROMs but also from CD-ROMs of older media. More precisely, it has CD-ROM readout compatibility, encouraging users to use newer media.
At present, so-called "high-definition DVDS" are being developed. So are high-definition DVD drives for reading out information from high-definition DVDs. A high-definition DVD has a higher recording density than a DVD-ROM. It is important for a high-definition DVD drive to have DVD-ROM readout compatibility, as the DVD drive has CD-ROM readout compatibility. That is, the high-definition DVD drive must be designed to read out information also from the older optical disks, i.e., DVD-ROMs, so that users can use not only the information recorded on high-definition DVDs but also the information recorded on the DVD-ROMs.
To enhance the recording density of an optical disk is based on reducing the diameter of a beam spot for reading out information from the optical disk. The diameter of the beam spot is proportional to the wavelength of the light beam emitted from the light source used. The diameter is inversely proportional to the numerical aperture (NA) of the objective lens which focuses the light beam on the surface of the optical disk. To increase the recording density of an optical disk, from the density of a CD-ROM to that of a DVD-ROM, the wavelength of the light beam emitted from the light source has been decreased from 780 nm to 650 nm, and the NA of the objective lens has been increased from 0.45 to 0.6. As a significant change between a CD-ROM and DVD-ROM, the substrate thickness of DVD-ROMs has been reduced from 1.2 mm to 0.6 mm.
DVD drives having CD-ROM readout compatibility can be classified into three types, in accordance with the number of light sources used and the number of objective lenses used. The first type has two light sources and one objective lens. The second type has one light source and two objective lenses. The third type has two light sources and two objective lenses. These three types of DVD drive certainly costs more than the type of one light source and one objective lens.
To reduce cost, another type of a DVD drive has been proposed which comprises one light source, one objective lens, and an aperture limitation element or a holographic optical element (HOE). The light source and the objective lens are used to read out information from both a DVD-ROM and a CD-ROM. The aperture limitation element or HOE is provided to correcting the aberration resulting from the difference in thickness between the DVD-ROM and the CD-ROM.
A high-definition DVD drive must have an extra optical element (either an aperture limitation element or a HOE) in order to have DVD-ROM readout compatibility, if the high-definition DVD substrate is thinner than a DVD-ROM. (This situation is similar to the DVD drive which needs to have such an extra optical element to correct aberration resulting from the difference in substrate thickness between the DVD-ROM and the CD-ROM.)
If the high-definition DVD has the same thickness as the DVD-ROM, the high-definition DVD drive can have DVD-ROM readout compatibility, without necessity of having such an extra optical element. In this case, an information-readout beam can form a beam spot on the recording surface of a DVD-ROM without causing a great aberration because the high-definition DVD has the same thickness as the DVD-ROM.
Even though the information-readout beam forms a beam spot on the recording surface of the DVD-ROM without causing aberration, however, high-quality signals may not be read out from the DVD-ROM if the pit depth of the DVD-ROM is inappropriate for the wavelength of the information-readout beam or if the pit width of the DVD-ROM is excessive with respect to the diameter of the beam spot. If the pit depth is inappropriate, the signals will have an insufficient amplitude. If the width is excessive with respect to the diameter of the beam spot, so-called "rebound" will occur in the signals corresponding to beams reflected from the long pits. These problems may be solved by use of particular optical elements. Use of such optical elements inevitably increases the manufacturing cost of the high-definition DVD drive. In view of this, it is desirable not to incorporate such optical elements in the high-definition DVD drive.
As mentioned above, a high-density optical disk drive (e.g., high-definition DVD drive) can form a beam spot on the recording surface of a low-density optical disk (e.g., DVD-ROM) without causing aberration, thereby to read out information from the low-density optical disk, even if it has no special optical elements, provided that the low-density optical disk has the same thickness as the high-density optical disk (e.g., high-definition DVD). However, if the pit depth is inappropriate for the wavelength of the light source, the signals will have an insufficient amplitude. If the pit width is excessive with respect to the diameter of the beam spot, the signals corresponding to beams reflected from the long pits will have an insufficient amplitude with "rebound". In either case, the high-density optical disk drive can hardly read out signals from the low-density optical disk.